Is there a specific reason for this? After all, it is "boys and girls", rather than "girls and boys". If the boy (male) comes first here, why doesn't it come first in "ladies and gentlemen"?
It probably has to do with the phonetic and metrical properties of "ladies and gentlemen" versus "gentlemen and ladies." Say them both out loud and see which one sounds better to you, intuitively.
The metrical pattern of "ladies and gentlemen" consists of (arguably) two dactyls. A dactyl is a group of three syllables where the first is stressed and the second two are unstressed. When these occur in succession, they are pleasant to the ear.
- ladies and gentlemen
- DUM da da / DUM da da
The metrical pattern of "gentlemen and ladies" falls into a much less regular pattern. It is less pleasant to the ear. It is something like a stressed syllable followed by three unstressed syllables followed by a stressed syllable followed by an unstressed syllable.
- gentlemen and ladies
- DUM da da da DUM da
Our language tends to organize itself according to regular and pleasant stress patterns. My guess is that is why we feel more drawn to "ladies and gentlemen" than "gentlemen and ladies" and why the former has conventionalized.
It comes from "My Lords, Ladies and Gentlemen". Titled men come first (My Lords); then their spouses [My] Ladies; Ladies also include non-titled ladies; and finally, untitled men (Gentlemen). Debretts, Preamble Precedents
The following list gives the form in which important guests should be included in a preamble in order of precedence:
Your Royal Highness
My Lord Mayor (My Lord Provost, etc) See (i)
Mr Recorder (outside London)
Mr Chairman of the ...... County Council (outside Greater London)
My Lord Chancellor
Prime Minister (or, more formally, Mr. Prime Minister). See (ii)
Your Excellency (ies) (this refers to Ambassadors and High Commissioners)
Your Grace(s). See (iii)
My Lord(s). See (iv)
Ladies and Gentlemen. See (v)
If you want to know what the footnotes (i) through (v) say, so you will be absolutely correct at your next speech, click on the link.